Aromatic polyesters generally represented by polyethylene terephthalate (PET) and polybutylene terephthalate (PBT) have been used extensively in the past as significant engineering plastics due to their excellent thermal resistance and other physical properties, such as mechanical strength and gas permeation resistance properties. Polybutylene terephthalate is not, however, typically used in end-use applications where high transparency is required since it has high crystallinity.
Transparent films of polyethylene terephthalate having low crystallinity can be relatively more easily produced by rapidly cooling molten PET because of its relatively low crystallization rate. However the crystallization of PET must be promoted by annealing procedures in order to improve its mechanical strength, gas permeation resistance and other similar properties. Annealing PET films, however, is problematic since elevated temperatures are typically employed in annealing procedures in order to enhance the crystallization efficiency. These elevated annealing temperatures tend to deleteriously affect the transparent properties of the film. That is, PET film sometimes becomes cloudy during annealing procedures at elevated temperatures due to the addition of nucleating agents which are employed to increase the resin's crystallization efficiency.
Spherulites are formed when molten polyethylene terephthalate resin is cooled slowly. The resulting resin product thus has high crystallinity due to the slow cooling, but has low transparency values due to scattering of visible light rays by the spherulites. What has been needed therefore are processes whereby polyester films having both high crystallinity and transparency properties may be produced.
Broadly, the present invention relates to a process for producing polyester film having high transparency values using a base polyester which is an aromatic copolyester containing specific comonomer units. The aromatic copolyester is then treated under specific film-forming conditions so as to obtain a film that is highly transparent.
More specifically, the present invention relates to processes for producing a transparent polyester film that are especially characterized by melt-blending a copolyester resin composition having structural units of the following general formulas (I) and (II): ##STR3## wherein R groups may be the same or different from each other and each represent a group selected from among --CH.sub.2 CH.sub.2, --CH.sub.2 CH.sub.2 OCH.sub.2 CH.sub.2 --, --CH--CH.sub.2 -- and ##STR4## and the molar fraction ratio of the structural unit (II) to the total of structural units (I+II) is between 0.02 to 0.20.
The melt-blended copolyester is then rapidly cooled (quenched) to provide a film which exhibits low crystallinity. The cooled film is then aged at a temperature of 60.degree. C. or below and is further heat-treated to increase crystallinity at a temperature between its glass transition point (T.sub.g) and no greater than 2.degree. C. below its melting point (T.sub.m) The transparent polyester film produced by the processes of the present invention will exhibit a crystallinity of at least 70%, and a haze value of 20% or less after heat-treatment at 120.degree. C.
The present invention thereby provides a process for producing a transparent film from a copolyester having the structural units represented by formulas (I) and (II), respectively, which generally comprises the steps of forming the copolyester into a sheet, rapidly cooling the sheet to impart low crystallinity thereto, aging it at a temperature of 60.degree. C. or below, and then subsequently heating it to a temperature between the glass transition point (T.sub.g) of the copolyester and no greater than 2.degree. C. below the copolyester's melting point.
Further aspects and advantages of the present invention will become more clear from the reader after carefully considering the detailed description of the preferred exemplary embodiments thereof which follows.